With reference to FIG. 1, the general architecture is recalled of a cellular mobile radiocommunications system such as, in particular, the Global System for Mobile communications (GSM). Such a system essentially comprises:
a set of base stations or "Base Transceiver Stations" ("BTSs"), such as those referenced BTS1, BTS2, BTS3, and BTS4; in the example shown, each of the base stations is assigned to a respective cell, the cells being referenced C1, C2, C3, and C4 (other examples naturally being possible in which the same BTS is assigned to a plurality of cells), and the base stations co-operate with mobile stations, such as MS1, MS2, MS3, MS4, situated in the cells; PA1 a set of "Base Station Controllers" ("BSCs"), such as those referenced BSC1 and BSC2, each of the BSCs controlling a respective subset of cells from the point of view of managing the radio resources; in the example shown, the Base Station Controller BSC1 controls the cells C1 and C2, and the Base Station Controller BSC2 controls the cells C3 and C4; and PA1 a set of "Mobile Switching Centers" ("MSCs"), such as that referenced MSC1, each of the MSCs co-operating with external networks, such as, in particular, Public Switched Telephone Networks (PSTNs) or Integrated Services Digital Networks (ISDNs), and controlling a subset of Base Station Controllers from the point of view of managing calls; in the example shown, the Mobile Switching Center MSC1 controls the Base Station Controllers BSC1 and BSC2. PA1 the server BSC, referenced BSC.sub.s, firstly sends a handover request message (referenced HO REQ in this example for "handover required") to the MSC; PA1 the MSC in turn sends a message of the same type (referenced HO REQ' in this example for "handover request") to the target BSC, referenced BSC.sub.c ; PA1 depending on whether or not a resource is available in the target cell, the target BSC sends to the MSC either, as shown in FIG. 2, a handover request acknowledgement message (referenced HO REQ ACK in this example) itself containing a handover command message, or else, as shown in FIG. 3, a message indicating that it is impossible to perform handover to said target cell (this message being referenced HO FAIL in this example for "handover failure"); and PA1 the MSC in turn sends a message of the same type to the server BSC, namely either a handover command message (referenced HO COMM in this example), or else a message indicating that it is impossible to perform handover to said target cell (this message being referenced HO REJ for "handover request rejected").
In such a system, a decision to hand over a call is generally taken for reasons of improving the transmission quality of the radio signals in the network, in particular so as to improve the quality of a call in progress (in particular to prevent the call from being cut off), or so as to improve the overall level of interference in the network.
In such a system, a decision to a hand over a call can also be taken for other reasons, in particular so as to overcome the problem of too much traffic in a cell, or because testing or maintenance operations are in progress in a cell. Under such conditions, handover may be said to be "forced" because it can go against the objects of the above-mentioned reasons of improving transmission quality.
In such a system, a decision to hand over a call is taken by the network on the basis of a certain number of parameters, such as, in particular, the results of measurements taken on radio signals for the current server cell and for cells neighboring the current server cell, and indications about the level of traffic in the cells and/or operator control data.
The cell to which the call is to be handed over may be referred to as the "new server cell", and it is further chosen from a set of cells referred to as "candidate cells" (determined on the basis of said measurement results) in the following manner.
Firstly a request is made to hand over the call to the best of the candidate cells. When such handover is possible, the best candidate cell constitutes the new server cell. When such handover is not possible, a request is made to hand over the call to the following candidate cell, in the order of preference of candidate cells, and so on.
A candidate cell to which handover is requested may also be referred to as the "target cell".
When the target cell is controlled by the same BSC as the current server cell (in FIG. 1, this situation corresponds, for example, to handover being requested from cell C1 to cell C2, or from cell C3 to cell C4, such handover also being referred to as "internal handover" or "intra-BSC handover"), the BSC in question is capable, by itself and internally, of determining whether or not handover to said target cell is possible, since the BSC is the network entity that is in charge of managing the radio resources both for the server cell and for the target cell.
When the target cell is controlled by a BSC (referred to as the "target BSC") other than the BSC (referred to as the "current server BSC" or more simply below as the "server BSC") that controls the current server cell (also referred to more simply below as the "server cells"), this situation corresponding for example, in FIG. 1, to a request to hand over a call from one or other of the cells C1 and C2 to one or other of the cells C3 and C4 (such handover also being referred to as "external handover", or "inter-BSC handover"), the procedure is as shown in one or other of FIGS. 2 and 3, namely:
As shown in FIGS. 4 and 5, once a handover (or "transfer") T.sub.1 from one cell C.sub.i to another cell C.sub.j has actually been performed, it is possible that the opposite handover T.sub.2 back to the initial server cell might be requested, in particular because the results of measurements of radio signals relating to the new server cell and to the initial server cell become more favorable to the initial server cell again, e.g. because the mobile station is situated at the limit of coverage between the two cells. That phenomenon is also known as the "ping-pong" phenomenon.
A problem that arises when the first handover is "forced", i.e. when the decision to perform such handover is taken for reasons other than those of improving the transmission quality of the radio signals in the network.
If such other reasons continue to exist at the time at which the opposite handover is requested, they are considered as taking priority over the reasons of improving the transmission quality of the radio signals in the network, which reasons lie behind said opposite handover, and, under such conditions, such an opposite handover must not be authorized.
As shown in FIG. 4, when both of the two cells in question are controlled by the same BSC, referenced BSC.sub.ij, the BSC is aware of the "forced" nature of the first handover, and is thus capable of deciding that such an opposite handover must not be performed.
As shown in FIG. 5, when the two cells in question are not controlled by the same BSC, the current server BSC (or "initial target BSC"), referenced BSC.sub.j, uses the procedure recalled above with reference to FIGS. 2 and 3 to send a handover request to the target BSC (or "initial server BSC"), referenced BSC.sub.i, and it then receives from the target BSC a message indicating that it is impossible to perform handover to the target cell (or "initial server cell").
In which case, an unnecessary handover request is made, thereby lengthening handover time unnecessarily, and giving rise to unnecessary consumption of transmission resources for the purpose of interchanging the corresponding signalling messages, and thus degrading the quality of service.